Browse Prior Art Database

USING SWITCHING MOSFETs IN LOW PHASE NOISE OSCILLATORS

IP.com Disclosure Number: IPCOM000007622D
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2002-Apr-10
Document File: 1 page(s) / 91K

Publishing Venue

Motorola

Related People

Eliav Zipper: AUTHOR [+3]

Abstract

Low phase noise is an important property of the synthesizer in HF SSB radio. The phase noise is depended on the phase noise of the VCO. Low phase noise VCOs are dificult to achieve and generally require high quality resonator elements. The active device, in the VCO, has a secondary importance in this aspect. Still, the experience that was gained dur- ing the development process shows, that JFET is superior to BJT for low phase noise VCOs.

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MOTOROLA Technical Developments

USING SWITCHING MOSFETs IN LOW PHASE NOISE OSCILLATORS

by Eliav Zipper, Yair Amran and Yehuda Eder

  Low phase noise is an important property of the synthesizer in HF SSB radio. The phase noise is depended on the phase noise of the VCO. Low phase noise VCOs are dificult to achieve and generally require high quality resonator elements. The active device, in the VCO, has a secondary importance in this aspect. Still, the experience that was gained dur- ing the development process shows, that JFET is superior to BJT for low phase noise VCOs.

  The effect of the active device on the VCO's phase noise is related to its ability to handle high level signals. The phase noise is always related to the signal level and therefore, an increase in the signal level at the VCO's loop is translated to an improvement of the phase noise. During the devel- opment of a Coplitz oscillator, a JFET device developed higher voltage on the resonator then the BJT The BJT is limited in this aspect since its base- emitter junction can handle a forward voltage of 1 V The JFET, can handle higher voltage levels. Another factor, that probably affects the phase noise improvement is the high input impedance of the JFET JFETs are known to have higher input imped- ance, compared to BJT In addition, the JFET's input impedance is not depended in the input voltage level, as the BJT's input impedance is. Since the Coplitz oscillator employs a parallel LC resonator, that at resonance has high impedance, the JFET is better matched to the resonator. These factors gave an 6 to 8 dB improvement in the oscillator's phase noise.

  JFETs introduces some drawbacks to VCOs. The limitation on the gate-source voltage, to be kept neg- ative, reduces the voltage amplitude at the gate. In the oscillator that was designed with a JFET, an additional diode was required to clip the gate-source voltage. Another drawback of the JFET is the high variance of its turn-off voltage (Vpp) and its capaci- tance (Cgs), that eliminates its performance in the aspect of robust design. The variance of Vpp caus...